Cold reduction mills are used throughout the steel industry for taking a coiled strip of hot-rolled, pickled steel and reducing the strip to the final gauge required by the customer. In a typical cold reduction tandem mill, the strip passes through a number of stands whereby each stand may reduce the strip by 20% to 25% in thickness. In a four-high tandem mill, each individual stand typically consists of a housing having two walls, a top, and a base that form an open window. Within the window is a vertical assembly of four rolls under pressure made up of two work rolls through which the strip passes and two backup rolls which help support the work rolls. The backup rolls and the work rolls are, in turn, supported in the individual stand by bearing assemblies known as chocks. In a four-high assembly, there are a total of eight chocks—four work roll chocks and four backup roll chocks—such that each chock supports each end of the four rolls. Each chock has two surfaces that face each of the two walls of the mill stand housing. The inner surface of the two walls of the mill stand housing and the chock surfaces facing those walls all possess metal liners to extend the life of each of the respective surfaces.
The strip to be rolled is fed from the entry side of the first mill stand, passes between the top and bottom work rolls, and then emerges from the exit side of the mill stand. In the same manner, the strip then proceeds through each successive stand in the tandem. Enough load is transferred to the strip via the rolls by means of a screw down or other type of pressure device situated atop each stand so that the strip emerges from the last stand in the sequence at the desired thickness.
Most cold reduction tandem mills, especially five and six stand cold reduction mills, operate at a high rate of speed, usually in the range of 4,000 to 6,000 feet per minute. At times, when operating at high speeds the cold reduction tandem mill may experience a condition known as third octave mode chatter, also referred to as audible and/or vibrational chatter. This type of chatter takes place when the two smaller work rolls are allowed to vary in separation or “bounce” in a short vertical direction at high movement frequency. The separation results when forces inherent to the rolling operation interact with the resonant characteristics of the mill housing. Vibrational chatter particularly affects high-speed, flat metal strip rolling mills.
There have been many theories put forth over the years regarding factors that contribute to chatter. These theories have focused on factors such as work roll and backup roll bearing wear, lubrication deficiencies, faulty work roll finish, strip interstand tensions, and directional forces exerted by the rolls, to name a few. Regardless of the contributing cause, in order for mill chatter to occur, there has to be a high frequency change in the work roll gap of the particular mill stand. This can only take place if there is a slight vertical movement between the two opposing work rolls. This problem is most prevalent on lighter gauge strip, and usually, if not always, on the later stands of a tandem mill.
Any horizontal movement of a back-up roll chock in relation to its mill stand housing liner, even if not excessive, can result in a slight vertical movement of the work rolls, resulting in vibrational-type chatter. In years past, experienced cold reduction mill operators tried to avoid chatter by driving metal shims between the backup roll chock liner and the mill window. This resulted in a crude and temporary, but sometimes effective, tool for reducing chatter.
Typical cold reduction mills are designed to have an initial clearance between a backup roll chock and its mill stand window of approximately 0.020 to 0.030 inches per side, or 0.040 to 0.060 inches total. The clearance is needed to facilitate changing and stacking of rolls and movement of spindles, couplings, and gears during operation. However, this intentionally designed initial gap quickly deteriorates over time because of vibrational forces, resulting in chatter and its accompanying problems. By eliminating the gap between the backup roll chock and the mill stand housing, the backup rolls are prevented from moving at a high frequency in the horizontal direction which, in turn, prevents the work rolls in the same stand from oscillating at a high frequency in the vertical direction, thus eliminating undesirable third octave mode chatter.
Chatter has long been a major quality and productivity issue for high-speed, cold reduction mills. Vibrational chatter can result in excessive gauge variation in the metal strip being produced. Chatter can cause undesirable, visible, ripple-like “chatter marks” along the strip, which can necessitate its rejection. In addition, if chatter is severe enough, strip breaks and equipment damage can occur, resulting in mill downtime and loss of productivity. To compensate for chatter, a mill usually has to reduce operating speed. It is not unusual for a high-speed, cold reduction mill to reduce its speed by 20 to 30% to avoid chatter. The steel coil that is produced when the mill is experiencing chatter often has to have the chatter-affected portion removed and downgraded to scrap, which necessitates additional reprocessing of the coil. This reprocessing and downgrading can cause the processor to incur substantial economic loss. Consequently, reduction or elimination of vibrational-type mill chatter results in higher mill speeds, greater productivity, fewer strip breaks, less reprocessing of defective product, less diverted product, less equipment damage, and most importantly for the processor, greater profitability.
There is known in the art numerous devices for adjustment of the gapping that develops between the chocks and the housing of a rolling mill stand. These devices typically employ some type of hydraulically activated means of taking up or compensating for the gap, usually in the form of pistons/cylinders or inflatable metal bladders which, when activated, either opposingly thrust against or expand outwardly into the chock/housing gap, thus reducing the opportunity for “play” or gapping and the resulting vibrational chatter.
Such hydraulically activated piston-like devices are described in U.S. Pat. Nos. 6,763,694 and 6,354,128, and U.S. patent application Ser. Nos. 10/433,758; 10/192,700; 10/192,641, 10/192,638, and 09/791,753. U.S. Pat. No. 4,402,207 describes a hydraulically activated bladder-type device.
With these devices, the adjustment means are situated either within the mill stand housing itself or incorporated into a movable structure separate from the chock and housing. When situating these devices in the mill stand housing, their installation and maintenance requires that the particular operating line completely shut down for extended periods, resulting in a loss in productivity. In addition, the machining and other modifications needed to install these devices within the mill housing could very likely compromise the housing's structural integrity. Further, installation of gap adjustment devices within a mill housing is limited to the particular stand involved, so that the specific device cannot easily be transferred to other stands or even other mills without expensive modification of the stand housing slated to receive the device. Similarly, location of hydraulically activated devices in separate support elements or movable frames requires that the chocks and housing be specially designed and fabricated to accommodate the additional structural element which itself may require extensive fabrication. Lastly, German patent DE 44 34 797 discloses a system of hydraulic pressure push rods inserted directly into roll chocks to correct the lie of the chocks. However, none of the above patents teach a practical and cost effective means of easily retrofitting existing rolling mill chocks to accommodate a commercially available means of providing horizontal thrust to take up the gapping between a chock and its mill housing to successfully eliminate vibrational chatter.
Because of the high cost involved, it would be rare for a rolling mill to purchase all new backup chocks solely for the purpose of fabricating and installing any of the devices and methods taught in the prior art. However, the present invention allows a mill to cost effectively retrofit existing backup chocks with commercially available materials to effectively eliminate vibrational chatter.